Method of assembling double-containment piping assemblies

ABSTRACT

A double-containment piping assembly has two anchor supports, and at least one elbow fitting with an outer elbow section and an inner elbow section received within the inner elbow section. A first leg having a first inner pipe received within a first outer pipe is coupled between an anchor support and the elbow fitting. A second leg having a second inner pipe received within a second outer pipe is coupled between the other anchor support and the other side of the elbow fitting. First flexibility supports are mounted on either side of the elbow fitting, and permit axial and lateral movement of the inner pipe relative to the outer pipe in the areas of the elbow. First axial-guiding supports, which permit axial movement of the inner pipe relative to the outer pipe, but prevent lateral movement of the inner pipe, are each mounted a minimum distance from the elbow fitting between the inner and outer pipes. Any differential expansion of the inner and outer pipes relative to each other accumulates at the elbow fitting, and is absorbed by movement of the inner elbow section relative to the outer elbow section.

This patent application is a divisional of U.S. patent application Ser.No. 08/089/745, filed Jul. 9, 1993, now U.S. Pat. No. 5,482,088.

FIELD OF THE INVENTION

The present invention relates to double-containment assemblies includinginner piping or conduits located within outer piping, conduits orcontainment components, and more particularly, to suchdouble-containment assemblies designed to permit movement of the innerpiping components and the outer containment components relative to eachother in response to events such as differential thermal expansion orcontraction.

BACKGROUND INFORMATION

There are numerous systems available providing a double ordual-containment assembly including inner or primary pipes containedwithin outer or secondary containment pipes to transport dangerous orhazardous fluids within the inner pipes. In the event of a leak oremission of fluid or vapors from the inner pipes, the leaking substanceis intended to be safely contained within the outer pipes. Historicalapplications for such double-containment systems are found in thenuclear, gas and petroleum refining, and chemical processing industries.It is also known to provide certain types of detectors and/or drainagedevices in the annulus between the inner and outer piping components inthe event that there is leakage, for the detection and/or collection ofsuch leakage.

Differential thermal expansion in double-containment systems occurs whenthe inner and outer piping components expand or contract to differentdegrees, or at different rates relative to each other. Almost alldouble-containment systems are subject to changes in temperature duringoperation, and/or to differences in operating temperature between theinner and outer piping components. This causes differential thermalexpansion or contraction of the inner and outer piping componentsrelative to each other, thus causing the inner and outer pipingcomponents to move relative to each other.

When the inner piping components expand or contract relative to theouter piping components, and if the inner piping components areinstalled in an axially unrestrained manner, the deflection of theprimary piping due to the expansion or contraction of the primary pipingaccumulates at the elbow sections of the primary piping. In this case,the inner elbow sections are subjected to bending and/or torsionalmovements relative to the outer elbow sections. Although elbow fittingsby their nature are capable of greater flexibility than comparablestraight sections of pipe, when elbow sections are subjected to bendingand/or torsional movements, stresses are intensified, and in someinstances, this intensification can lead to failure.

In many known double-containment systems, the inner piping componentsare not permitted to bend or otherwise move either laterally and/oraxially relative to the outer piping components, particularly in thearea of the elbows. Rather, the straight sections of inner piping arerestrained by interstitial supports or other types of fittings withinthe outer piping components, and/or the inner elbow sections arerestrained by fittings relative to the outer elbow sections or areattached to the outer elbow sections, preventing movement of the innerand outer elbow sections relative to each other.

In some known double-containment systems, the inner piping componentsmay be able to move axially relative to the outer piping components, butthe elbow fittings do not permit movement of the inner elbow sectionsrelative to the outer elbow sections. The elbow sections become pointsof restraint, which can lead to failure when there is differentialthermal expansion or contraction.

In other known double-containment systems, the inner piping componentsmay be able to move relative to the outer piping components, but onlywithin narrow limits, and once these narrow limits are exceeded, theinner piping components come into contact with the outer pipingcomponents. This is frequently the case when standard or off-the-shelfshort-radius/short-radius combinations of elbow fittings are employed tomake double-containment elbow fittings, which has been a common practiseto date in the double-containment industry. Because the centerlineradius of the inner elbow section is less than the centerline radius ofthe outer elbow section, there is less space between the larger-radiussurfaces than between the smaller-radius surfaces within the annulusbetween the inner and outer elbow sections. As a result, when the mostcommon types of differential thermal expansion or contraction occur,i.e., when the inner piping expands toward the outer piping or when theouter piping contracts toward the inner piping, there is contact betweenthe inner and outer piping components. The elbow fittings are thereforenot permitted to fully bend or flex in response to differential thermalexpansion or contraction, but rather essentially behave as internalanchors, and become points of restraint, which frequently leads topremature failure.

It is typically necessary in double-containment piping systems toprovide support for the primary piping by positioning one or moreinterstitial supports between the primary and outer containment sectionsof straight pipe, thus employing the structural integrity of the outercontainment piping to support the primary piping through suchinterstitial supports. Typically, multiple interstitial supports areused, and the spacing between the interstitial supports is selectedbased on the longest span of primary piping that can be allowed beforethe primary piping sags or deflects beyond a maximum allowabledeflection. The degree of deflection of the primary piping depends uponthe weight of the primary piping, the weight of the fluid transportedthrough the primary piping, the internal pressure and temperature of theprimary piping, the material of construction of the primary piping, andthe amount of temperature change experienced during operation of theprimary piping. The temperature change is determined based on thetemperature condition of the double-containment pipe assembly at thecompletion of construction in comparison to the high or low temperaturesthat it will experience when in service.

As described above, almost all chemical and petroleum product pipingsystems are subject to changes in temperature during operation, andduring such temperature changes, there can be relatively substantialexpansion and/or contraction of the primary piping relative to the outercontainment piping causing the inner piping to move axially, radiallyand/or laterally relative to the outer containment piping. Typically,the interstitial supports in known double-containment systems do notaccommodate for such relative movements, or the selection and/orlocation of such interstitial supports within the double-containmentsystems does not adequately compensate for such relative movements. As aresult, significant stress is induced within such double-containmentsystems, which frequently can lead to a rupture or other failure of theprimary piping or outer containment piping.

Accordingly, in double-containment systems developed to date, there hasbeen insufficient means (and in many instances no means) foraccommodating or alleviating differential thermal expansion and/orcontraction of the inner and outer piping components relative to eachother, and as a result, such systems have operated as restrainedsystems, developing large axial stresses, which can lead to failure, andleakage of hazardous fluids or vapors.

SUMMARY OF THE INVENTION

The present invention is directed to a double-containment assembly,comprising a first anchor support, and at least one elbow fittingincluding an inner elbow section contained within an outer elbowsection, and defining an unobstructed annulus between the inner andouter elbow sections permitting movement of the inner elbow sectionrelative to the outer elbow section. A first inner pipe section iscoupled between the first anchor support and the inner elbow section,and a first outer pipe section is coupled between the first anchorsupport and the outer elbow section. A first axial-guiding support isspaced a first predetermined distance from the at least one elbowfitting for supporting the first inner pipe section within the firstouter pipe section, and includes means for permitting axial movement ofthe first inner pipe section and first outer pipe section relative toeach other, and for substantially preventing lateral movement of thefirst inner pipe section relative to the first outer pipe section. Afirst flexibility support is spaced a second predetermined distance fromthe elbow fitting less than the first predetermined distance, forsupporting the first inner pipe section within the first outer pipesection, and includes means for permitting axial and lateral movement ofthe first inner pipe section relative to the first outer pipe section.

In one embodiment of the present invention, the double-containmentassembly further comprises a second anchor support, and a second innerpipe section coupled between the second anchor support and the innerelbow section on the opposite side of the inner elbow section relativeto the first inner pipe section. A second outer pipe section is coupledbetween the second anchor support and the outer elbow section on theopposite side of the outer elbow section relative to the first outerpipe section. A second axial-guiding support is spaced a thirdpredetermined distance from the elbow fitting for supporting the secondinner pipe section within the second outer pipe section, and includesmeans for permitting axial movement of the second inner pipe section andsecond outer pipe section relative to each other, and for substantiallypreventing lateral movement of the second inner pipe section relative tothe second outer pipe section. A second flexibility support is spaced afourth predetermined distance measured from the elbow fitting, which isless than the third predetermined distance, for supporting the secondinner pipe section within the second outer pipe section, and includesmeans for permitting axial and lateral movement of the second inner pipesection relative to the second outer pipe section.

The present invention is also directed to a method of assembling adouble-containment assembly, comprising the steps of: a) selecting thelocation of a first anchor support relative to a first elbow fitting,and coupling first inner and outer pipe sections between the firstanchor support and corresponding first inner and outer elbow sections ofthe first elbow fitting; b) coupling second inner and outer pipesections to the other side of each inner and outer elbow section; c)determining an expected overall change in linear dimension due totemperature changes for i) the first inner and outer pipe sections, andii) the second inner and outer pipe sections; d) comparing the expectedoverall change in linear dimension for each of the first inner and outerpipe sections, and second inner and outer pipe sections to the distancein the axial directions of each of the first and second pipe sectionsbetween the inner and outer elbow sections; and e) selecting an elbowfitting with sufficient space between the inner and outer elbow sectionsin the axial direction of either the first or second pipe sections toaccommodate the expected change in linear dimension of each pipesection.

One embodiment of the present invention further comprises the steps ofdetermining a first minimum distance of a first axial-guiding supportmounted between the first inner and outer pipe sections from the elbowfitting, wherein the first minimum distance is based on the expectedoverall change in linear dimension due to temperature changes of thesecond inner and outer pipe sections, and installing the firstaxial-guiding support at a location greater or equal to the firstminimum distance if the distance between the elbow fitting and the firstanchor support is greater than the first minimum distance.

Preferably, the method of the present invention further comprises thestep of mounting a first flexibility support between the first inner andouter pipe sections a second minimum distance from the elbow fitting,wherein the second minimum distance is less than the first minimumdistance.

One advantage of the present invention, is that any differential thermalexpansion or contraction of the inner piping components and outer pipingcomponents relative to each other accumulates at the elbow fittingbetween the internal anchors. The elbow fitting absorbs the differentialthermal expansion or contraction by movement of the inner elbow sectionand outer elbow section relative to each other, without contacting eachother. The flexibility supports are mounted adjacent to the elbowfittings to permit axial, lateral, and if necessary, radial movements ofthe inner piping relative to the outer piping in the areas of theelbows. The axial-guiding supports permit only axial movement of theinner piping relative to the outer piping to substantiallyconcentrically guide the inner piping through the outer piping, andensure that the distortion of the inner piping remains within designlimits. Each of the piping components in the assembly of the presentinvention, is thus selected and located to accommodate differentialthermal expansion or contraction of the inner and outer pipingcomponents relative to each other, without permitting the inner andouter piping to contact each other, which can lead to failures normallyassociated with prior double-containment assemblies.

Other advantages of the present invention will become apparent in viewof the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial schematic, cross-sectional view a double-containmentassembly embodying the present invention.

FIG. 1A is a plan view of a typical flexibility support employed in thedouble-containment assembly of FIG. 1.

FIG. 1B is a plan view of a typical axial-guiding support employed inthe double-containment assembly of FIG. 1.

FIG. 2 is a flow chart illustrating conceptually the procedural stepsfor constructing a double-containment assembly in accordance with thepresent invention.

FIG. 3 is a partial schematic, cross-sectional view of anotherembodiment of a double-containment assembly of the present inventionincluding an offset leg.

FIG. 4 is a partial schematic, cross-sectional view of anotherembodiment of a double-containment assembly of the present inventionincluding an expansion loop.

FIG. 5 is a partial schematic, cross-sectional view of anotherembodiment of a double-containment assembly of the present inventionincluding an expansion lop oriented substantially in a vertical plane.

FIG. 6 is a partial schematic, cross-sectional view of anotherembodiment of a double-containment assembly of the present inventionincluding only straight sections of piping coupled between intermediateanchors.

DETAILED DESCRIPTION

In FIG. 1, a double-containment assembly embodying the present inventionis indicated generally by the reference numeral 10. Thedouble-containment assembly 10 comprises a plurality of sections ofinner or primary pipe contained within outer secondary or containmentpipe, including a first section of inner pipe 12 coupled to a secondsection of inner pipe 14 by an inner elbow section 16. The first innerpipe section 12 is contained within a first outer pipe section 18, andthe second inner pipe section 14 is contained within a second outer pipesection 20. The outer pipe sections 18 and 20 are coupled together by anouter elbow section 22 containing the inner elbow section 16, andforming an elbow fitting indicated generally by the reference numeral24.

The elbow fitting 24 is preferably of the type disclosed in theco-pending U.S. patent application filed on the same date as the presentpatent application in the name of Christopher G. Ziu, entitled "ElbowFittings With Expanded Outer Annulus Space For Double-ContainmentAssemblies", filed Jul. 9, 1993 and accorded Ser. No. 08/089,798 nowU.S. Pat. No. 5,452,922, which is hereby expressly incorporated byreference as part of the present disclosure. As can be seen in FIG. 1,there is a larger distance between the larger-radius surfaces thanbetween the smaller-radius surfaces of the inner elbow section 16 andthe outer elbow section 22, defining an expanded or relatively wideouter annulus space 26 in comparison to a more narrow inner annulusspace 28. In most instances of differential thermal expansion orcontraction of the inner and outer piping components relative to eachother, the inner piping components expand and in turn cause the innerelbow section to move outward toward the outer elbow section, or theouter piping components contract and in turn cause the outer elbowsection to move inward toward the inner elbow section. In either case,the relative movement of the inner and outer piping components occurswithin the outer annulus space, and the relatively wide outer annulusspace 26 is sufficient to accommodate such movement without permittingthe inner and outer elbow sections to contact each other, and possiblylead to failure, as is often the case with prior double-containmentassemblies.

Other types of elbow fittings may also be used in the double-containmentassemblies of the present invention; for example, the elbow fittingsdescribed in co-pending patent U.S. application Ser. No. 07/859,278,filed Mar. 26, 1992, entitled "Elbow Fittings For Double ContainmentPipe Assemblies", which is a continuation of U.S. application Ser. No.07/681,324, filed Apr. 4, 1991, now abandoned, which is hereby expresslyincorporated by reference as part of the present disclosure. These elbowfittings also suitably permit movement of the inner piping and innerelbow section and the outer piping and outer elbow section relative toeach other to accommodate differential thermal expansion or contraction.In these fittings, the curved portion of the inner elbow section isconcentric with the curved portion of the outer elbow section, defininga substantially uniform and unobstructed space throughout the annulusbetween the inner and outer elbow sections to permit the necessarymovement of the inner and outer elbow sections relative to each other inresponse to differential thermal expansion or contraction.

The first inner pipe section 12 and first outer pipe section 18 are eachcoupled on the end opposite the elbow fitting 24 to an intermediateinternal anchor support, which in this embodiment of the presentinvention is a first termination fitting 30. Similarly, the second innerpipe section 14 and second outer pipe section 20 are each coupled on theend opposite the elbow fitting 24 to a second internal anchor support ortermination fitting 32. The termination fittings 30 and 32 each act asan internal anchor, preventing movement of the inner and outer pipingcomponents relative to each other in the direction of the respectivetermination fitting. Thus, any expansion or contraction of the first andsecond inner and outer pipe sections is permitted to occur between thefirst and second termination fittings 30 and 32, respectively.Accordingly, if either the first or second inner pipe sections 12 or 14undergo expansion, the growth of the inner pipes accumulates at theelbow fitting 24, and is absorbed at the elbow fittings by movement ofthe inner elbow section 16 outward toward the outer elbow section 22within the expanded outer annulus space 26. Similarly, if there iscontraction of the outer piping components relative to the inner pipingcomponents, the decrease in length of the outer piping is absorbed bythe elbow fitting 24, by movement of the outer elbow section 22 inwardtoward the inner elbow section 28 within the outer annulus space 26. Asshown in FIG. 1, the termination fittings 30 and 32 each terminate theannulus space between the respective inner and outer piping componentsby means of a solid annulus portion located between the inner and outerflanges of the fitting. In many instances, however, flow passages areformed through the annulus portion to permit the flow of fluid or vaporsthrough the fittings into an annulus on the other side of each fitting.

The internal anchors 30 are preferably of the type disclosed in U.S.Pat. No. 5,141,261, dated Aug. 25, 1992, entitled "Double ContainmentPipe Joint Assembly", or of the type disclosed in U.S. patentapplication Ser. No. 07/934,705, filed Aug. 24, 1992, entitled "DoubleContainment Pipe Joint Assembly", which is a continuation-in-part ofpatent application Ser. No. 07/681,331, filed Apr. 4, 1991, now U.S.Pat. No. 5,141,261, each of which is hereby expressly incorporated byreference as part of the present disclosure. Another type ofintermediate anchor that may equally be employed as the fittings 30and/or 32 is disclosed in U.S. Pat. No. 5,085,471, dated Feb. 4, 1992,entitled "Double Containment Pipe Joint Assembly", which also herebyexpressly incorporated by reference as part of the present disclosure.Many of these preferred internal anchors are not termination fittings,but rather include flow passages for the flow of fluid within theannulus through the fittings.

The double-containment assembly 10 also includes a plurality offlexibility supports 34 supporting the first inner pipe section 12within the first outer pipe section 18 and the second inner pipe section14 within the second outer pipe section 20. The first flexibilitysupport 34 on each side of the elbow fitting 24 is spaced apredetermined distance away from the elbow fitting, and the successiveflexibility supports 34 are substantially equally spaced a predetermineddistance relative to each other on the other side of the firstflexibility support relative to the elbow fitting 24. The flexibilitysupports 34 permit axial, lateral, and if necessary, they can bedesigned to permit radial movement of the inner piping componentsrelative to the outer piping components. This ability to permit relativemovement of the inner piping components in both the lateral and axialdirections (and if necessary, in radial directions) is particularlyadvantageous in the areas adjacent to the elbow fittings to adequatelyaccommodate differential thermal expansion and contraction of the innerand outer piping components relative to each other.

The flexibility supports 34 are preferably of the type disclosed inco-pending patent application Ser. No. 07/722,083, filed Jun. 27, 1991,now U.S. Pat. No. 5,197,518, dated Mar. 30, 1993, entitled "CenteringSupport Assembly For Double Containment Pipe Systems", and U.S. patentapplication Ser. No. 08/037,083, filed Mar. 25, 1993, entitled"Centering Support Assembly For Double Containment Pipe Systems", whichis a continuation-in-part of patent application Ser. No. 07/722,083, nowU.S. Pat. No. 5,197,518, which are each hereby expressly incorporated byreference as part of the present disclosure.

A typical flexibility support 34 is illustrated in FIG. 1A, and includesa base portion 35, a pair of stanchion portions 37 formed on either sideof the base portion 35, and a cross-piece portion 39 extending betweenthe other ends of the stanchion portions 37. A saddle portion 41 ismounted on the base portion 35, and defines a curved surface for seatingthe inner piping 12 or 14. The saddle portion 41 is movable relative tothe base portion 35 between the stanchion portions 37 to permit lateralmovement of the inner piping relative to the outer piping. If necessary,the cross-piece portion 39 is spaced above the inner piping a distancesufficient to permit radial movement of the inner piping relative to theouter piping. In other instances, the cross-piece portion 39 is spacedclosely to the inner piping to prevent or permit only limited radialmovement of the inner piping, which may be necessary to control bucklingof the inner piping during operation of the double-containment assembly.This embodiment of the flexibility support 34 is only exemplary,however, and as is made evident in the above-described patentapplications, there are numerous other embodiments of flexibilitysupports that can adequately permit axial, lateral, and if necessary,radial movements of the inner piping components relative to the outerpiping components in the double-containment assemblies of the presentinvention.

Beyond a predetermined distance from the elbow fittings, such as theelbow fitting 24, the use of the flexibility supports 34 may have onlyminimal effect, and beyond this distance, axial-guiding interstitialsupports 36 which restrict lateral movement of the inner piping andfunction as axial guides are used instead. The axial-guiding supports 36are spaced a predetermined distance relative to each other, and arepreferably equally spaced from the first axial-guiding supportrelative-to the elbow fitting to the respective intermediate anchor 30or 32, as is described further below.

The axial-guiding supports 34 are preferably of the type disclosed inco-pending U.S. patent application Ser. No. 07/885,670, filed Aug. 17,1992, entitled "Centering Support For Double-Containment Pipe Assembly",now U.S. Pat. No. 5,404,914, and which is hereby expressly incorporatedby reference as part of the present disclosure. A typical axial-guidingsupport 36 is illustrated in FIG. 1B, and includes a base portion 43defining a first curved surface 45 for seating the inner piping 12 or14, and a clamp portion 47 defining a second curved surface 49 forengagement with the substantially opposite side of the inner pipingrelative to the first curved surface 45. As shown in FIG. 1B, theaxial-guiding support 36 includes a pair of fasteners for attaching theclamp portion 47 to the base portion 45 to fix the support to the innerpiping.

Each axial-guiding support 36 is moveable relative to the outer pipingto permit axial movement of the inner piping relative to the outerpiping. However, because each axial-guiding support 36 is secured to theinner piping, and extends between the inner piping and the outer piping,the support prevents lateral and/or radial movement of the inner pipingrelative to the outer piping in the area of the respective support. Asshown in FIG. 1B, the axial-guiding support 36 includes a semi-circularcutout in the clamp portion 47, and a flat chord cut in the base portion45 to permit fluid flow, if necessary, through the annulus between theinner and outer piping. Each axial-guiding support 36 may also include alayer of elastomeric material between each of the curved surfaces 45 and49 and the inner piping to facilitate in dampening any vibrationalmovements of the inner piping.

Other types of axial-guiding supports that may be employed as thesupports 36 are disclosed in co-pending U.S. patent application Ser. No.08/088,864, now U.S. Pat. No. 5,400,828, entitled "Double-ContainmentPiping Supports For Improved Annulus Flow", filed on Jul. 8, 1993, whichis assigned to the same also hereby expressly incorporated by referenceas part of the present disclosure.

These types of axial-guiding supports are only exemplary, and as isevident in the above-identified patent applications, there are numerousembodiments of axial-guiding supports that permit axial movement of theinner and outer piping components relative to each other, but preventlateral and/or radial movements of the inner piping relative to theouter piping, which may be employed in the double-containment assembliesof the present invention.

In FIG. 2, a flow chart illustrates conceptually the procedural steps inconstructing the double-containment assemblies in accordance with thepresent invention. The first step is to determine the locations of theintermediate anchors, which in FIG. 1 are the termination fittings 30and 32, as indicated as S1 in FIG. 2. Typically, the intermediateanchors are located approximately at the mid-point of each respectivestraight section of piping, or half-way between elbow fittings, or otherdirectional changes in the double-containment assembly. This may notalways be the case, however, depending upon any unusual requirements ofa particular system. As shown in FIG. 1, the first termination fitting30 is located a distance L1 from the centerline of the second inner andouter pipe sections 14 and 20, and the second termination fitting 32 islocated a distance L2 from the centerline of the first inner and outerpipe sections 12 and 18.

Once the intermediate anchor locations are selected (L1 and L2), this isthe starting point for determining the locations of the interstitialsupports within the double-containment assembly. The next step is todetermine the expected overall changes in dimension due to thermalexpansion or contraction of the straight sections (or legs) on eitherside of the elbow fitting, as indicated by S2 in FIG. 2. The expectedchange in linear dimension of each respective straight pipe section isdetermined based on the following equation:

    ΔL=C×ΔT×L                          (1)

wherein ΔL is the expected change in linear dimension of the respectiveinner or outer straight section of pipe;

C is the coefficient of thermal expansion of the respective inner orouter section of pipe, which is based on the material of construction ofthe pipe;

ΔT is the expected change in temperature of each respective section ofpipe during operation of the double-containment assembly 10; and

L is the length of the respective straight section of inner or outerpipe, i.e., the distance from the face of the intermediate anchor 30 or32 to the centerline of the inner and outer straight sections on theother side of the elbow fitting.

The expected change in temperature (ΔT) for each inner section of pipeis typically based on the expected change in temperature of the fluidpassed through the inner pipe, or the change in temperature when fluidis being transported within the inner piping as compared to thetemperature of the system at installation and/or when fluid is not beingtransported. With respect to the outer sections of piping, the expectedchange in temperature (ΔT) is typically based on the expected change inthe surrounding or ambient temperature of the outer piping from theinstallation temperature, which is subject to greater fluctuations inaboveground, outdoor systems, as opposed to underground systems.

For aboveground systems, in which the outer piping is not restrainedrelative to the inner piping, the overall change in linear dimension(ΔL) for each leg of the double-containment assembly is determined byseparately calculating the change in linear dimension of the respectiveinner straight pipe section, and the change in linear dimension of therespective outer straight pipe section in accordance with equation (1)above, and the difference between the two is the overall change inlinear dimension of that leg (ΔL).

For underground systems, in which the outer piping components arerestrained relative to the inner piping components, or abovegroundsystems in which the outer piping components are restrained relative tothe inner piping components, the change in linear dimension of the outerpiping sections due to thermal expansion or contraction is de minimis,if not zero. Therefore, in these systems, the overall change in lineardimension (ΔL) for each leg of the double-containment assembly iscalculated using equation (1) above for only the inner piping sections.

Once ΔL is determined for each leg of the double-containment assembly,the next step is to determine whether there is sufficient space providedin the outer annulus 26 of the elbow fitting 24 to accommodate theexpected overall thermal expansion or contraction of each leg coupled tothe elbow fitting, as indicated by S3 in FIG. 2. In other words, theoverall change in linear dimension (ΔL) for each of the straight pipesections must be less than the space provided in the outer annulus 26 inthe axial direction of the piping in order to accommodate such thermalexpansion or contraction, without permitting the inner and outer elbowsections to contact each other, which could lead to a rupture in thepiping. If the outer annulus space 26 is insufficient to accommodate theexpected overall thermal expansion or contraction (ΔL), then the nextstep is to select a new elbow fitting 24 which provides a sufficientlywide outer annulus space 26 to accommodate the overall ΔL, as indicatedby S4.

The next step is to determine the location of each of the firstaxial-guiding supports 36 with respect to the elbow fitting 24, asindicated by S5 in FIG. 2. The location of the first axial-guidingsupport 36 on the right side of the elbow fitting 24 in FIG. 1 isindicated as L3, which is the distance from the centerline of the innerand outer straight sections of pipe 14 and 20 to the mid-point of therespective first axial-guiding support. The location of the firstaxial-guiding support 36 on the left side of the elbow fitting 24 inFIG. 1 is indicated as L4, which is the distance from the centerline ofthe inner and outer straight sections of pipe 12 and 18 to the mid-pointof the respective first axial-guiding support. The distances L3 and L4are each determined based on the following equation:

    L= (ΔL×OD×10.sup.6)/1.6S!.sup.1/2        (2)

wherein L is either L3 or L4, i.e., the distance from the centerline ofthe straight sections of pip on the opposite side of the elbow fittingto the mid-point of the respective axial-guiding support;

ΔL is the expected overall change in linear dimension of the inner andouter straight pipe sections on the opposite side of the elbow fitting24, as determined above based on equation (1);

OD is the outside diameter of the inner pipe; and

S is the maximum desired stress for the elbow fitting 24. As an example,for an elbow fitting of ferrous-based material, the maximum allowablestress is within the range of approximately 10,000 to 30,000 psi.

The distances L3 and L4 determined in accordance with equation (2) abovedefine the minimum distances from the elbow fitting at which the firstaxial-guiding supports 36 can be located. It may be desirable to locateeach first axial-guiding support 36 at a distance greater than L3 or L4,respectively, depending upon the requirements of a particular system.For example, in order to have substantially equal spacing between theaxial-guiding supports, it may be desirable to locate the firstaxial-guiding support at a distance greater than L3 or L4, dependingupon the distance between L3 or L4 and the respective internal anchor.It is not recommended, however, to locate either of the firstaxial-guiding supports closer to the elbow fittings than the positionsdefined by the distances L3 or L4.

Once the distances L3 and L4 are determined, the next step is todetermine the locations of the additional axial-guiding supports 36between each first axial-guiding support 36 and the adjacent terminationfitting 30 or 32, as indicated by S6 in FIG. 2. The additionalaxial-guiding supports 36 are substantially equally spaced a distance L5relative to each other based on the following equations:

    L= (76.9Y×E×I)/W!.sup.1/4                      (3a)

or

    L= (185.2Y×E×I)/W!.sup.1/4                     (3b)

    or

    L= (144.9Y×E×I)/W!.sup.1/4                     (3c)

or

    L= (153.8Y×E×I)/W!.sup.1/4                     (3d)

wherein:

L is the spacing between adjacent axial-guiding supports (L5 in FIG. 1);

Y is the mid-span vertical displacement or sag due to the weight of theinner piping (a span is a section of pipe between supports);

E is the modulus of elasticity of the inner piping material (psi) at themaximum expected inner-piping temperature;

I is the moment of inertia of the respective inner pipe section (in⁴);and

W is the unit weight of the inner piping when filled with the fluid thatis being transported.

One of equations 3a through 3b is selected based on the number of spanswithin a respective straight pipe section, wherein one span is definedas one section of pipe between points of support. Equation (3a) isemployed if there is only one span; equation (3b) is employed if thereare two spans; equation (3c) is employed if there are three spans; andequation (3d) is employed if there are four or more spans.

The next step is to determine the location of each of the firstflexibility supports 34 with respect to the elbow fitting 24, asindicated by S7 in FIG. 2. The first flexibility support 34 on the rightside of the elbow fitting 24 in FIG. 1 is located a distance L6 from thecenterline of the straight pipe sections on the opposite side of theelbow fitting, and the first flexibility support 34 on the left side ofthe elbow fitting in FIG. 1 is located a distance L7 from the centerlineof the straight pipe sections on the opposite side of the elbow fitting.The distances L6 and L7 are selected so that the sum of L6 and L7(L6+L7) is less than or equal to approximately three-quarters of L5(0.75×L5). Typically, L6 and L7 are substantially equal; however, incertain instances one may be greater than the other, depending upon theunique considerations of a particular system.

The other flexibility supports 34 are substantially equally spacedrelative to the first flexibility supports 34 or each other a distanceL8, between each first flexibility support and the respective firstaxial-guiding support 36, as shown in FIG. 1 and indicated as step S8 inFIG. 2. The distance L8 is determined in accordance with equations(3a)-(3d) above, and is equal to L5.

One advantage of the double-containment assemblies constructed inaccordance with the procedural steps of the present invention, is thatthe system is inherently flexible in order to accommodate movements ofthe inner and outer piping components relative to each other in responseto differential thermal expansion or contraction, or other effectscausing movements of the inner and outer piping components relative toeach other. Each of the fittings and supports is suited to accommodatespecific types of movements of the piping components relative to eachother, and the location of each fitting or support is selected in orderto adequately support the piping components, yet to permit movements ofthe inner and outer piping components relative to each other and avoidthe failures normally associated with prior art, restraineddouble-containment systems.

The intermediate anchors control the direction of expansion orcontraction of the piping components so that the changes in lineardimensions of the straight pipe sections accumulate at the elbowfittings located between the termination fittings. The elbow fittingsare uniquely designed to in turn accommodate the movements of the innerand outer piping components relative to each other by permittingdisplacement of the inner and outer elbow sections relative to eachother without contacting each other. The flexibility supports arelocated adjacent to the elbow sections to accommodate the greater extentof relative movements in the areas of the elbow fittings, because theflexibility supports permit axial, lateral, and if desired, radialmovements of the inner piping components relative to the outer pipingcomponents, while also supporting the inner piping components within theouter piping components. The axial-guiding supports are located betweenthe flexibility supports and the intermediate anchors to permit axialmovement of the inner and outer piping components relative to each otherto accommodate linear expansion or contraction of the inner and outerpiping components. The axial-guiding supports also substantially preventother movements of the inner and outer piping components relative toeach other in order to guide the piping into the elbow section to ensureproper compensation.

Turning to FIG. 3, another double-containment assembly embodying thepresent invention is indicated generally by the reference numeral 110.The double-containment assembly 110 includes the same components as theassembly 10 described above in connection with FIG. 1, and thereforelike reference numerals preceded by the numeral 1 are used to indicatelike elements. The double-containment assembly 110 is an offsetassembly, including an offset leg 140 coupled between two elbow fittings124. The offset leg 140 is a relatively short span of straight pipe, andis substantially shorter in length than either of the straight sections(or legs) located between the other side of each elbow fitting 124 and arespective termination fitting 130 or 132. Because the offset leg 140 isrelatively short in length, it does not include any axial-guidingsupports like the axial-guiding supports 136. Rather, the onlyinterstitial supports located within the offset leg 140 are flexibilitysupports 134, permitting axial, lateral, and if desired, radialmovements of the inner piping components relative to the outer pipingcomponents. Based on equation (2) above, there is insufficient distancebetween the two elbow fittings 124 to mount an axial-guiding supportlike the supports 136 between the elbow fittings.

In this type of offset embodiment of the present invention, whendetermining the locations of the first axial-guiding supports 136 withrespect to the adjacent elbow fittings 124, equation (2) above isemployed. However, the change in linear dimension (ΔL) used to determinethe distance L in equation (2) is the change in linear dimension for thestraight pipe section on the other side of the respective elbow fittingemploying axial-guiding supports, i.e., the leg on the other side of theoffset leg 140. The location of each first flexibility support 134within the offset leg 140 with respect to the adjacent elbow fitting 124is determined in the same manner as described above in connection withthe flexibility supports 34 illustrated in FIG. 1.

In FIG. 4, another double-containment assembly embodying the presentinvention is indicated generally by the reference numeral 210. Thecomponents employed within the double-containment assembly 210 are thesame as the components employed in the double-containment assembly 110described above and illustrated in FIG. 3, and therefore like referencenumerals preceded by the numeral 2 instead of the numeral 1 are used toindicate like elements. The double-containment assembly 210 differs fromthe double-containment assembly 110 in that instead of an offsetconstruction, the double-containment assembly 210 forms an expansionloop 142 between two sections of straight pipe. The expansion loop 142is formed by four elbow fittings 224 and three expansion legs 144coupled between the four elbow fittings. As can be seen, each expansionleg 144 is relatively short in length in comparison to the otherstraight sections of pipe, and none of the expansion legs are longenough (based on equation (2) above) to accommodate an axial-guidingsupport like the axial-guiding supports 36 described above andillustrated in FIG. 1. The length L_(A) of each of the two parallel legs144 of the expansion loop 142 is determined based on the followingequation:

    L.sub.A = (0.75×ΔL×E×D)/S!.sup.1/2

wherein:

L_(A) is the length of each of the two parallel legs of the expansionloop.

ΔL is the overall change in the linear dimension of the adjacent legleading into the expansion loop due to expected thermal expansion orcontraction calculated in accordance with equation (1) above, i.e., theoverall change in linear dimension of the respective straight section ofpiping on the other side of the respective elbow fitting. Forunderground systems, the overall change in linear dimension is theoverall change in linear dimension of the inner straight section ofpiping. For aboveground systems, wherein the outer piping is notconstrained with respect to the inner piping, the overall change inlinear dimension is the difference between the change in lineardimension of the outer straight section of piping and the change inlinear dimension of the inner straight section of piping.

E is the modulus of elasticity of the piping material at the coolestexpected temperature of operation of the piping for which the expectedΔL is determined.

D is the outside diameter of the piping for which the expected ΔL isdetermined.

S is the maximum desired working stress at operating temperatures of thepiping for which the expected ΔL is determined.

The length L_(B) of the expansion leg 144 extending between the twoparallel expansion legs 144, as shown in FIG. 4, is preferably equal toapproximately one-half of L_(A) (0.5×L_(A)).

The location of each flexibility support 234 within the expansion loop142 is preferably determined in the same manner as with the flexibilitysupports 34 described above in connection with FIG. 1. The distance L1between the flexibility supports 234 is determined by employing equation(3) above, and the location of each first flexibility support withrespect to the adjacent elbow fitting 224, e.g., L2 and L3 in FIG. 4, isdetermined so that the sum of L2 and L3 (L2+L3) is less than or equal tothree-fourths of L1 in FIG. 4 (L2+L3)≦(0.75×L1).

With double-containment assemblies including an expansion loop asillustrated in FIG. 4, it is desirable to have little, if any lateral orradial movement of the inner straight pipe sections 212 and 214 relativeto the outer straight pipe sections 218 and 220 in the areas adjacent tothe elbow fittings 224. Instead, it may be desirable to havesubstantially all bending absorbed by the elbows 224 and legs 144 of theexpansion loop. Accordingly, it is not necessary to employ theflexibility supports 234 on either side of the expansion loop 142 (asopposed to within the expansion loop) when there is no need toaccommodate lateral or radial movement of the inner piping in this area.Thus, in many such instances, the flexibility supports 234 are optionalin these areas, although a flexibility support may be used instead of anaxial-guiding support, if desired.

The distance X of the first axial-guiding support 236 with respect toeach elbow fitting 224 (or the location of the first flexibility support234 used instead of an axial-guiding support), which is the lineardistance measured from the centerline of the adjacent expansion leg 144,is selected so that it is no greater than approximately 4 times thediameter of the inner piping. The second axial-guiding support 236 withrespect to each elbow fitting is spaced a distance Y from the firstaxial-guiding support, which is no greater than approximately 14 timesthe diameter of the inner piping. The remaining axial-guiding supports236 located between the second axial-guiding support 236 and therespective intermediate anchor 230 or 232 are substantially equallyspaced relative to each other by a distance calculated in accordancewith equations (3a)-(3d) above.

The expansion loop of the double-containment assembly 210 isadvantageous for accommodating differential thermal expansion orcontraction between the inner and outer piping components of anotherwise straight double-containment assembly. As described above, eachof the four elbow fittings 224 is adapted to absorb differential thermalexpansion or contraction of the inner piping components relative to theouter piping components by movement of the inner elbow section relativeto the outer elbow section within the expanded outer annulus space 226.The expansion loop is therefore adapted to accommodate the differentialthermal expansion or contraction of the piping components locatedbetween the intermediate anchors 230 and 232. By employing these typesof expansion loops, offsets, and/or elbow fittings in accordance withthe present invention, the outer diameter of the piping components canbe minimized, resulting in space and material cost savings, while stillaccommodating differential thermal expansion or contraction of the innerand outer piping components relative to each other.

Turning to FIG. 5, another double-containment assembly embodying thepresent invention is indicated generally by the reference numeral 310.The double-containment assembly 310 employs many of the same componentsas the double-containment assembly 210 described above in connectionwith FIG. 4, and therefore like reference numerals preceded by thenumeral 3 instead of the numeral 2 are used to indicate like elements.Like the double-containment assembly 210 described above, thedouble-containment assembly 310 includes an expansion loop formed byfour elbow fittings 324 and three expansion legs 344 coupled between thefour elbow fittings. This embodiment of the present invention differsfrom the embodiments described above, in that the expansion loop islocated in a substantially vertical plane, whereas the expansion loopdescribed above is located in a generally horizontal (or inclined, ifnecessary) plane. Thus, the center expansion leg 344 is orientedsubstantially horizontal, whereas the two parallel, side expansion legsare oriented substantially vertical. As can be seen, the verticalexpansion legs 344 do not include any flexibility supports 334 (or anyother type of interstitial supports), whereas the horizontal expansionleg 344 includes flexibility supports 334 substantially equally spacedrelative to each other in the same manner as the flexibility supports234 employed in the expansion loop of the double-containment assembly210 described above. When determining the factors based on the weight ofthe inner piping of the horizontal expansion leg, however, it isappropriate to include the weight of the inner piping of each of thevertical expansion legs, which are also supported by the flexibilitysupports 334 in the horizontal expansion leg.

Turning to FIG. 6, another double-containment assembly embodying thepresent invention is indicated generally by the reference numeral 410.The double-containment assembly 410 includes many of the same componentsas the double-containment assemblies described above, such as thedouble-containment assembly 10 illustrated in FIG. 1, and therefore likereference numerals preceded by the numeral 1 are used to indicate likeelements.

In this embodiment of the present invention there are no directionalchanges, such as offsets or expansion loops, but rather only straightpipe sections coupled between intermediate anchors, which in thisembodiment of the present invention are termination fittings 430. Afirst flexibility support 434 is located approximately at the mid-pointbetween the two termination fittings 430, and the other flexibilitysupports 434 are substantially equally spaced a distance L relative toeach other between the first flexibility support 434 and each of thetermination fittings 430. The spacing L is determined based on equations(3a)-(3d) above in the same manner as described for determining thespacing between the flexibility supports 34 in FIG. 1. In thisembodiment of the present invention the flexibility supports 434 areadvantageously employed between fixed anchor points to permit theprimary piping to deflect between the fixed anchor points into asubstantially sinusoidal shape, for example, as illustrated in FIG. 6,to accommodate differential thermal expansion of the inner pipingcomponents relative to the outer piping components.

In the embodiment of the present invention illustrated in FIG. 6 it maybe possible to also employ axial-guiding supports, like theaxial-guiding supports 36 described above in connection with FIG. 1,located between the flexibility supports 434 and the terminationfittings 430, if there is sufficient distance between the terminationfittings to accommodate such supports. Equation (1) above is employed todetermine the distance from the first flexibility support 434(preferably located approximately at the mid-point between thetermination fittings 430) and the first axial-guiding support spacedaway from the first flexibility support. If the distance between thefirst flexibility support and a respective termination fitting isgreater than the calculated distance for the first axial-guidingsupport, then the axial-guiding supports may be mounted between theflexibility supports and the termination fittings in the same manner asdescribed above for the axial-guiding supports 36 illustrated in FIG. 1.

I claim:
 1. A method of assembling a double-containment piping assemblyhaving a first internal anchor, an elbow fitting having an outer elbowsection defining an axis, and an inner elbow section defining an axisfor receipt within the outer elbow section to thereby form an annularspace between the inner and outer elbow sections, and at least one pairof inner and outer pipe sections having an elongated outer pipe section,and an elongated inner pipe section for receipt within the elongatedouter pipe section and defining an annulus between the inner and outerpipe sections, the method comprising the steps of:determining anexpected overall change in elongated dimension due to temperaturechanges for the at least one pair of inner and outer pipe sections;selecting an elbow fitting with sufficient annular space between theinner and outer elbow sections in the elongated direction of the pair ofelongated inner and outer pipe sections to accommodate the expectedchange in elongated dimension of the inner and outer pipe sectionswithin the annular space; and assembling the elongated inner pipesection within the elongated outer pipe section and coupling the innerand outer pipe sections between the first internal anchor and one end ofthe selected elbow fitting such that at least one of the inner and outerelbow sections is movable relative to the other in at least its axialdirection within the annular space and is movable relative to the firstinternal anchor to thereby accommodate the expected overall change inelongated dimension of the pair of elongated inner and outer pipesections in response to temperature changes.
 2. A method as defined inclaim 1, wherein the double-containment piping assembly has a first pairof elongated inner and outer pipe sections and a second pair ofelongated inner and outer pipe sections, the method further comprisingthe steps of:determining an expected overall change in elongateddimension due to temperature changes for i) the first pair of elongatedinner and outer pipe sections, and ii) the second pair of elongatedinner and outer pipe sections; selecting an elbow fitting withsufficient annular space between the inner and outer elbow sections inthe elongated direction of each of the first and second pairs of innerand outer pipe sections to accommodate the expected change in elongateddimension of each pair of inner and outer pipe sections within theannular space; assembling the first elongated inner pipe section withinthe first elongated outer pipe section and coupling the first elongatedinner and outer pipe sections between the first internal anchor and oneend of the selected elbow fitting; and assembling the second elongatedinner pipe section within the second elongated outer pipe section andcoupling the second elongated inner and outer pipe sections to the otherend of the selected elbow fitting such that at least one of the innerand outer elbow sections is movable relative to the other in at leastits axial direction within the annular space to thereby accommodate theexpected overall change in elongated dimension of each pair of elongatedinner and outer pipe sections in response to temperature changes.
 3. Amethod as defined in claim 2, further comprising the steps of (i)determining a minimum distance between a first axial-guiding support anda predetermined point on the elbow fitting based on the expected overallchange in elongated dimension due to temperature changes of the secondinner and outer pipe sections, and (ii) if the distance between theelbow fitting and the first internal anchor is greater than said minimumdistance, installing a first axial-guiding support at a location greaterthan or equal to said minimum distance between the first inner and outerpipe sections.
 4. A method as defined in claim 3, further comprising thesteps of mounting additional axial-guiding supports between the firstaxial-guiding support and the first internal anchor, and substantiallyequally spacing the additional axial-guiding supports relative to eachother.
 5. A method as defined in claim 3, further comprising the step ofdetermining the minimum distance between the first axial-guiding supportand the predetermined point on the elbow fitting based on (i) theexpected overall change in elongated dimension due to temperaturechanges of the second inner and outer pipe sections, (ii) an outsidediameter of the respective inner pipe section, and (iii) a maximumdesired stress for the elbow fitting.
 6. A method as defined in claim 3,further comprising the step of mounting at least one additionalaxial-guiding support between the first axial-guiding support and thefirst internal anchor, and spacing each such additional axial- guidingsupport a predetermined distance from an adjacent axial-guiding supportbased on (i) an approximate mid-span vertical displacement due to theweight of the respective inner pipe section, (ii) a modulus ofelasticity of the respective inner pipe section, (iii) a moment ofinertia of the respective inner pipe section, and (iv) a unit weight ofthe respective inner pipe section loaded with a fluid to be transported.7. A method as defined in claim 3, wherein the first axial-guidingsupport includes means for permitting axial movement of the first innerpipe section relative to the first outer pipe section, and means forpreventing at least one of lateral and radial movement of the firstinner pipe section relative to the first outer pipe section.
 8. A methodas defined in claim 3, further comprising the step of mounting a firstflexibility support between the first inner and outer pipe sectionsanother minimum distance from said predetermined point on the elbowfitting, wherein the minimum distance of the first flexibility supportis less than the minimum distance of the first axial-guiding support. 9.A method as defined in claim 8, further comprising the steps of mountingadditional flexibility supports between the first flexibility supportand the first internal anchor, and substantially equally spacing theflexibility supports relative to each other.
 10. A method as defined inclaim 8, further comprising the steps of coupling a second internalanchor to an opposite end of the second inner and outer pipe sectionsrelative to the elbow fitting; determining a minimum distance from apredetermined point on the elbow fitting of a second axial-guidingsupport to be mounted between the second inner and outer pipe sectionsbased on the expected overall change in elongated dimension due totemperature changes of the first inner and outer pipe sections; and ifthe distance between the elbow fitting and the second internal anchor isgreater than said minimum distance, installing the second axial-guidingsupport at a location greater than or equal to said minimum distance.11. A method as defined in claim 10, further comprising the step ofmounting a second flexibility support between the second inner and outerpipe sections another minimum distance from said predetermined point onthe elbow fitting, wherein the minimum distance of the secondflexibility support is less than the minimum distance of the secondaxial-guiding support.
 12. A method as defined in claim 11, furthercomprising the step of mounting at least one additional axial-guidingsupport between the first axial-guiding support and the first internalanchor and between the second axial-guiding support and the secondinternal anchor.
 13. A method as defined in claim 12, wherein the sum ofthe minimum distance for the first flexibility support and the minimumdistance for the second flexibility support is less than or equal toapproximately 75% of the distance between adjacent axial-guidingsupports.
 14. A method as defined in claim 12, wherein each suchadditional axial-guiding support is spaced a predetermined distance froman adjacent axial-guiding support based on (i) an approximate mid-spanvertical displacement due to the weight of the respective inner pipesection, (ii) a modulus of elasticity of the respective inner pipesection, (iii) a moment of inertia of the respective inner pipe section,and (iv) a unit weight of the respective inner pipe section loaded witha fluid to be transported.
 15. A method as defined in claim 1, furthercomprising the step of determining the expected overall change inelongated dimension due to temperature changes for the at least one pairof elongated inner and outer pipe sections based on (i) the coefficientof thermal expansion of the respective inner or outer pipe section, (ii)the expected change in temperature of the respective inner or outer pipesection, and (iii) the length of the respective inner or outer pipesection.
 16. A method as defined in claim 1, wherein at least one of theelongated inner and outer pipe sections is unrestrained relative to theother, and the expected overall change in elongated dimension due totemperature changes for such pair of inner and outer pipe sections isdetermined by initially determining the expected change in elongateddimension of the inner pipe section and separately determining theexpected change in elongated dimension of the outer pipe section, andthen determining the difference between the expected change in elongateddimension for the inner and outer pipe sections.
 17. A method as definedin claim 1, wherein the outer pipe section is restrained relative to theinner pipe section, and the expected overall change in elongateddimension due to temperature changes for such pair of inner and outerpipe sections is determined based on the expected change in elongateddimension of the inner pipe section only.
 18. A method as defined inclaim 1, further comprising the step of mounting a flexibility supportbetween the first inner and outer pip sections a minimum distance from apredetermined point on the elbow fitting, wherein the flexibilitysupport comprises means for permitting at least one of axial, radial andlateral movement of the first inner pipe section relative to the firstouter pipe section.
 19. A method as defined in claim 1, wherein thefirst internal anchor includes means for preventing expansion of atleast one of the first inner pipe section and the first outer pipesection relative to the other in a direction toward the first internalanchor.
 20. A method as defined in claim 1, wherein the inner elbowsection and the outer elbow section define an unobstructed annulusbetween them for permitting axial, lateral and radial movement of atleast one of the inner and outer elbow sections relative to the other.21. A method as defined in claim 1, wherein the double-containmentpiping system further includes an offset leg assembled to the oppositeend of the elbow fitting relative to the pair of elongated inner andouter pipe sections, the method further comprising the stepsof:assembling a second elongated inner pipe section within a secondelongated outer pipe section and coupling one end of the second innerand outer pipe sections to another end of the elbow fitting; determiningan expected overall change in elongated dimension due to temperaturechanges for a third pair of elongated inner and outer pipe sections;selecting a second elbow fitting having an outer elbow section definingan axis and an inner elbow section defining an axis for receipt withinthe outer elbow section, wherein the second inner and outer elbowsections define an annular space between them which is sufficient in theelongated direction of the third pair of elongated inner and outer pipesections to accommodate the expected overall change in elongateddimension of the third pair of inner and outer pipe sections within theannular space; assembling the selected second inner elbow section withinthe selected second outer elbow section and coupling one end of thesecond elbow fitting to another end of the second elongated inner andouter pipe sections; assembling the third elongated inner pipe sectionwithin the third elongated outer pipe section and coupling one end ofthe third inner and outer pipe sections to another end of the secondelbow fitting such that at least one of the second inner and outer elbowsections is movable relative to the other in at least its axialdirection within the annular space to thereby accommodate the expectedoverall change in elongated dimension of the third pair of elongatedinner and outer pipe sections in response to temperature changes.
 22. Amethod as defined in claim 1, further comprising the steps of forming anexpansion loop by:assembling a first expansion leg having a first innerexpansion piping section received within a first outer expansion pipingsection, and connecting one end of the first expansion leg to anotherend of the elbow fitting; assembling a second elbow fitting having asecond inner elbow section received within a second outer elbow section,and connecting one end of the second elbow fitting to the other end ofthe first expansion leg; assembling a second expansion leg having asecond inner expansion piping section received within a second outerexpansion piping section, and connecting one end of the second expansionleg to another end of the second elbow section such that at least one ofthe second inner and outer elbow sections is movable relative to theother in at least its axial direction within the annular space tothereby accommodate movement of the first and second expansion legs inresponse to temperature changes; assembling a third elbow fitting havinga third inner elbow section received within a third outer elbow section,and connecting one end of the third elbow fitting to the other end ofthe second expansion leg; assembling a third expansion leg having athird inner expansion piping section received within a third outerexpansion piping section, and connecting one end of the third expansionleg to another end of the third elbow section such that at least one ofthe third inner and outer elbow sections is movable relative to theother in at least its axial direction within the annular space tothereby accommodate movement of the second and third expansion legs inresponse to temperature changes; assembling a fourth elbow fittinghaving a fourth inner elbow section received within a fourth outer elbowsection, and connecting one end of the fourth elbow fitting to the otherend of the third expansion leg; and assembling a second elongated innerpipe section within a second elongated outer pipe section, andconnecting one end of the second elongated inner and outer pipe sectionsto another end of the fourth elbow fitting such that at least one of thefourth inner and outer elbow sections is movable relative to the otherin at least its axial direction within the annular space to accommodatemovement of the third expansion leg and second elongated inner and outerpipe sections in response to temperature changes.